Lignocellulosic/thermoplastic composites are materials that combine thermoplastic polymers with lignocellulosic materials, which are used as either reinforcements or fillers. The advantages of such composites are well documented in prior art such as in Canadian Patent Application No. 2,527,325 (Sain et al.).
There are numerous sources of lignocellulosic materials that may be used as thermoplastic composite fillers or reinforcements. Such sources may include both wood and non-wood based materials. Non-wood based materials may include bast and leaf fibres taken from agricultural crop such as hemp, flax, wheat, and sisal.
There are also various mechanically or chemically processed forms of supplying such lignocellulosic materials depending on the source. Such forms include: powders or particulates including wood flour and sawdust, chopped fibres or strands, continuous rovings, woven and non-woven mats, and pulp.
Pulp fibres are generally a product of lignocellulosic materials that have been processed through a combination of chemical and/or mechanical pulping processes. Such processes including Kraft and Mechanical pulping are well known within the pulp and paper industry. Although lignocellulosic pulp can be produced from agricultural fibre sources, by far the largest source of pulp in the world is wood for use in papermaking, paperboard, and absorbent products applications.
Most lignocellulosic pulps are densified and packaged into a baled form, which represents a low-cost method of storing and transporting such raw materials. More specifically, bales of commercially available wood pulp represent a large and relatively reliable supply of lignocellulosic fibres for use in thermoplastic composites applications.
There are numerous conventional processes that are used to combine lignocellulosic materials with thermoplastics, most of which are well known to those skilled in the art of plastics processing. Such processes include extrusion, compounding, compression molding, injection molding, and combinations or variations thereof.
To date, the majority of conventional and commercially available lignocellulosic-thermoplastic composites use lignocellulosic materials that are supplied in the form of powders such as wood flour. As such, feeding difficulties into such manufacturing processes are reduced, as powders are better flowing than fibrous materials. The other major category of conventional lignocellulosic/thermoplastic composite materials includes compression-molded parts, however in this case feeding issues are diminished as the fibres are typically placed into an open mold and are supplied in various forms of mats.
The feeding of fibrous materials into plastics molding or compounding machines such as extruders are well-known challenges in the thermoplastic composites field. If the bulk density of the material is too low, or if the fibre lengths are too long, “bridging” occurs. “Bridging” refers to a case where the material does not flow, and is in effect clogging transition points in the process, such as at the feed throat of an extruder.
In the fields of both glass and lignocellulosic fibre reinforced thermoplastics, numerous approaches have been used to solve problems of feeding and bridging. Many solutions to feeding fibrous materials are also linked with other technical requirements, including mechanical performance.
An increased fibre length is often desired in thermoplastic composite applications to maximize mechanical properties. However, the flowability of fibrous materials decreases with increasing fibre lengths. One solution to this problem is to change the nature of the process and provide fibrous material in the form of yarns or continuous rovings. In this case the rovings are “pulltruded” and subsequently cut and mixed through an extruder. The disadvantage of such a process is that producing rovings or yarns increases the raw material costs.
Barlow et al. in U.S. Pat. No. 6,743,507 (2004) describe a process in which lignocellulosic pulp is first converted into a densified pellet using a water-soluble binder for ease of feeding. It requires that the fibres in sheet form be first broken into discrete bundles, which in turn are pelletized with the binder. The addition of both these steps and the use of binding chemicals would add cost to the process. Another embodiment of the invention involves capturing and pelletizing the pulp at the mill before it is formed into sheets. This embodiment does not allow for the use of commercially available market pulp.
Dezutter et al. in U.S. Pat. No. 6,811,879 disclose a new form of flake pulp having a specific size, density and wet dispersibility, that may be metered in specified quantities when adding to cementitious products due to the fact that bulk quantities of the flakes flow well in conduits and other enclosed containers. Dezutter et al. in U.S. Pat. No. 6,837,452 further discloses a process for dewatering liquid pulp stock to produce singulated pulp flakes. Such flakes may be sent to a baler for packaging.
Dezutter and Hansen in U.S. Pat. Nos. 7,201,825 and 7,306,846 disclose a process for making discrete particles of cellulosic material that are flowable and meterable. Such particles comprise singulated cellulose fibres that have been densified.
Process aids may be added to assist in the flow of fibrous materials. Khavkine et al. in U.S. Pat. No. 6,883,399 (2004) incorporate a blend of flax bast fibres and shives for improved flowability. Shimada et al. in U.S. Pat. No. 5,087,518 (1992) combine a mixture of glass flakes and fibres to provide for a free-flowing reinforcing material in thermoplastic resins.
Despite the numerous sources and forms of lignocellulosic materials available, and the existing techniques of feeding fibrous materials, there is a need for a process capable of supplying densified lignocellulosic pulps into a variety of thermoplastic composite manufacturing processes. The process should meet the particular requirements of such composite manufacturing processes including supplying pulp without any feeding difficulties, and should enable precise control of the feed rates.
The present invention meets the aforementioned requirements and is capable of delivering lignocellulosic pulp into composite manufacturing processes without any bridging effect. It allows for the use of conventional market pulp, is readily scalable, and is capable of integrating directly into any thermoplastics composite manufacturing process, whether operating in continuous or batch modes.